The present invention relates to the calibration of level (gain) and frequency response in recording and transmission systems, particular at audio frequencies. More particularly, the invention relates to apparatus and methods for accurately and quickly accomplishing such calibration or the confirmation of prior calibration by comparison, particularly auditory, of a reference signal with a test signal played back or reproduced from the recording or transmission system.
The invention provides an arrangement for ensuring that the played back signal from a storage medium, such as magnetic tape or an optical (photographic, cinematographic) sound track, or from a transmission channel, such as a radio-frequency, telephone, or fiber optic link, matches in level and frequency response the signal applied to the storage medium or to the transmission channel.
For convenience, the invention will be described in connection with a magnetic tape recorder. In the preferred embodiment, the invention is associated with noise reduction companding circuitry, although the invention may or may not operate at the same time the noise reduction circuitry is active. The invention is not limited to use in magnetic tape recorders nor need it be associated with noise reduction systems.
It is known to provide various types of test signals for the purpose of calibrating and aligning recording systems, transmission systems and channels, sound playback systems, and the like. For example, well known procedures for aligning magnetic tape recorders typically include the following or similar steps:
(1) Align the recorder playback gain and frequency response by use of a standard reference tape on which a series of sine waves at known levels and discrete frequencies throughout the audio range are recorded;
(2) Using an oscillator and blank tape, and measuring signals at the output of the playback process, set the recording bias for a desired characteristic of recording distortion and/or noise and/or high frequency compression;
(3) Adjust the recording gain while measuring the playback output, using a sine wave at a mid-frequency (e.g., 1 kHz); and
(4) Align the frequency response of the recording process at the frequency extremes by applying test signals (discrete tones throughout the spectrum and/or sweep tones and/or pink noise), recording and playing back such test signals and modifying the recording equalization characteristics as may be necessary.
In the past ten years pink noise has been introduced by Dolby Laboratories as a test signal to align the optical sound heads and preamplifiers of motion picture projectors. The signal is applied to a real time analyzer (spectrum analyzer) to provide a quicker indication of the system level and frequency response than would be possible using a multiplicity of spot frequency tones. In recording studios, on the other hand, such alignment procedures continue to be performed using discrete frequencies even though they need to be performed on a frequent and regular basis, usually daily and/or when a new batch of tape is put into use. However, because the procedures are time consuming and boring, proper calibration is often not done and in any event is not subject to a rapid confirmation by the users of the equipment (during recording sessions, for example). Moreover, few recording studios own the real time analyzers which are necessary for pink noise alignment techniques and few studio technicians and recording engineers are familiar with pink noise techniques.
The present invention provides an inexpensive, quick, and easy to use pink noise calibration system that will be used frequently and thereby ensure an overall higher quality of the recording (transmission) and playback (reproduction) process.
Such an improvement in recording/transmission technology is useful in its own right but is also particularly valuable when compression/expansion noise reduction systems are used. In such systems, the levels in the noise reduction decoder (expander) should be as close as possible to those in the noise reduction encoder (compressor). One prior art arrangement, described in U.S. Pat. No. 3,760,102, applies a fixed frequency tone having a standard level to tapes for that purpose. The tone is frequency modulated so as to uniquely identify the tone without affecting its standard level.